In electrical machinery, for example electric motors which are pivotally supported by a bearing, there is the danger that current will flow through the bearing and that the associated spark erosion will lead to bearing damage. Current flow through the bearing can be caused by the voltages on the terminals of the electric motor being coupled to the rotor of the electric motor via stray capacitances, for example out of the stator windings. The associated current flows ultimately cause voltage to form on the bearing, which voltage can be greater than the breakdown strength of the lubricating film in the bearing and thus can trigger spark erosion. The situation may be of particular concern in electric motors operated with frequency converters because especially high pulsed voltage characteristics occur on the terminals relative to the motor housing and the rotor. Relatively high voltages thus also occur on the bearing.
Generally speaking, bearings are protected against unwanted current passage by electrical insulation provided to prevent current passage. This can have a relatively high associated cost, however, depending on the special circumstances of the application. Moreover, DC electrical insulation of the bearing does not always constitute sufficient protection. Thus, for example, for high frequency parasitic currents there is the danger that coupling into the bearing will take place capacitively.
In this regard, the technical article “High Frequency Leakage Current Reduction Based on a Common-Mode Voltage Compensation Circuit” in the IEEE journal, 1996, pages 1961-1967, describes compensating high frequency leakage currents using a compensation circuit. The wiring is designed such that compensation occurs in the power path, i.e. upstream of the connected load. This has the disadvantage that the compensation circuit is incorporated into the power circuit and thus its components must be designed for comparatively high wattages. Moreover any reactive component which has been inserted into the power circuit, such as for example an inductance, a capacitance or a filter, increases the number of resonant frequencies, for which, when excited, unpredictable overvoltages can occur. Since, when using a frequency converter, it can be largely assumed that all these resonant frequencies can be excited, there is a high risk that such an overvoltage will occur.